Merge pull request #37058 from BerriAI/litellm_passthrough_accept_encoding

fix(passthrough): stop forwarding client Accept-Encoding upstream

litellm/passthrough/utils.py

@@ -18,6 +18,7 @@ _PASS_THROUGH_PROTECTED_HEADERS: Final[frozenset] = frozenset(
         "x-goog-api-key",
         "host",
         "content-length",
+        "accept-encoding",
     }
 )
 
@@ -69,6 +70,9 @@ class BasePassthroughUtils:
             # Header We Should NOT forward
             request_headers.pop("content-length", None)
             request_headers.pop("host", None)
+            # accept-encoding must stay client-negotiated: forwarding e.g. "br" when
+            # the brotli package is absent relays undecodable bytes to the caller
+            request_headers.pop("accept-encoding", None)
 
             custom_header_names: Final = {header_name.lower() for header_name in headers}
             for header_name in list(request_headers.keys()):

tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py

@@ -568,6 +568,32 @@ def test_forward_headers_custom_wins_case_insensitive_over_request_authorization
     assert result["x-request-id"] == "req-123"
 
 
+def test_forward_headers_never_forwards_client_accept_encoding():
+    """
+    The client's Accept-Encoding must not reach the upstream provider: the proxy's
+    HTTP client decodes the upstream body and advertises only encodings it can
+    decode. Forwarding e.g. "br" on an install without the brotli package makes
+    the proxy relay raw compressed bytes with the content-encoding header stripped
+    (garbled JSON for /v1/models and count_tokens through the Anthropic passthrough).
+    """
+    from litellm.passthrough.utils import BasePassthroughUtils
+
+    request_headers = {
+        "accept-encoding": "gzip, deflate, br, zstd",
+        "x-pass-accept-encoding": "br",
+        "x-request-id": "req-123",
+    }
+
+    result = BasePassthroughUtils.forward_headers_from_request(
+        request_headers=request_headers,
+        headers={},
+        forward_headers=True,
+    )
+
+    assert "accept-encoding" not in result
+    assert result["x-request-id"] == "req-123"
+
+
 @pytest.mark.asyncio
 async def test_vertex_passthrough_custom_model_name_replaced_in_url():
     """